1. Field of the Invention
The present invention relates to a micro-heater and a sensor incorporating the micro-heater.
2. Description of the Related Art
The micro-heater disclosed in JP-A-11-271123 (corresponding to U.S. Pat. No. 6,450,025) is a conventionally used micro-heater in which a thin film heating portion is formed across cavity provided in a silicon substrate.
The thin film heating portion is constructed by laminating a tensile stress film made of silicon nitride, a heater layer and a compressive stress film made of silicon oxide. In this manner, the tensile stress and the compressive stress cancel, to thereby release the internal stress of the micro-heater.
Another micro-heater is disclosed in JP-A-11-194043 (corresponding to U.S. Pat. No. 6,240,777). In this micro-heater, both a diaphragm layer and a coating layer are formed to have a small tensile stress.
3. Problems to be Solved by the Invention
In the micro-heater disclosed in JP-A-11-271123 (corresponding to U.S. Pat. No. 6,450,025), the tensile stress component of the internal stress becomes small depending on the film thickness ratio of the compressive stress film to the tensile stress film. As the temperature of the atmosphere, in which the micro-heater operates, rises to a high temperature (e.g., 400° C. or higher), the heater element constituting the heater layer, due to thermal expansion of its forming material, tends to warp the laminated film structure of the tensile stress film, the heater layer and the tensile stress film. As a result, the micro-heater is subjected to thermal stress so as to disadvantageously cause thermal damage. In the micro-heater disclosed in JP-A-11-194043 (corresponding to U.S. Pat. No. 6,240,777), in which the diaphragm layer and the coating layer impart more or less tensile stress, the laminated film structure also becomes warped when the temperature of the atmosphere in which the micro-heater operates exceeds 400° C., to thereby invite a similar disadvantage. Moreover, no consideration is taken in JP-A-11-194043 (corresponding to U.S. Pat. No. 6,240,777) as to the area of the laminated film in a top plan view. As the area of the laminated film becomes large, another problem arises in that the more or less tensile stress of JP-A-11-194043 (corresponding to U.S. Pat. No. 6,240,777) results in a laminated film having insufficient strength (or shock resistance).
In order to cope with the above problems, an object of the present invention is to provide a micro-heater, which prevents, in advance, warpage of a laminated film comprising a tensile stress film and a compressive stress film, even if a heater element embedded in the laminated film is thermally expanded, and which has improved shock-resistance, and a sensor incorporating the micro-heater.